How can you set, if at all possible, the channels for OpenVDB export?
well, as far as I know there is just basic vdb support and blender does not offer you the possibilities houdini does. the vdb data is hard coded and depends on the sim settings. Though I have experienced that blender exported vdbs hold all the information needed to shade them within houdini. If you feel like the same simulation exports differently every time this is probably a bug. If it changes together with certain sim settings I would think it is supposed to be that way.